Think Your Pet has a Food Allergy? Eliminating Mistakes in Elimination Diet Trials

by Lisa M. Freeman, DVM, PhD, DACVIM (Nutrition)

Although food allergies are quite uncommon, they can sometimes occur in dogs and cats that have year-round skin issues (not seasonal) or chronic gastrointestinal problems.  Unfortunately, there’s no easy way to diagnosis them.  Blood or saliva tests may be appealing as an easy way to diagnose food allergy, but they are highly inaccurate so don’t waste your money on them!  The only way to diagnose a food allergy is with elimination diet trial.  To do an elimination diet trial, you must feed your pet ONLY a veterinary diet specifically designed for elimination diet trials for the prescribed period of time.

Before you consider doing an elimination diet trial to diagnose food allergy, you should know up front that they are not easy (you’ll see why as you read on), so you don’t want to go through it more than once. Most veterinary specialists recommend an elimination diet trial of at least 8-12 weeks for pets with skin issues and 3-4 weeks for those with digestive issues.  Therefore, my philosophy is to make sure that the trial is carefully done with the right diet, for the right length of time, and without making common mistakes.  That way, if your pet’s symptoms improve, then your pet may have a food allergy (there are more steps to take), but if your pet doesn’t get significantly better during an elimination diet trial, food allergy is even less likely to be the cause of your pet’s problem and you can move on to other, more likely causes of skin or digestive issues.

Elimination diet options

There are two main approaches to the diet used for an elimination diet trial:

1. Novel ingredient diets

A novel ingredient diet is one that contains ingredients your pet has never eaten before.  They aren’t inherently less allergenic – they’re just novel to that pet and therefore won’t trigger an allergy – so it’s important to be aware that what is novel for one dog may not be novel for another.  When considering what ingredients your pet has eaten before, it’s important to consider not only the main protein source (meats, etc), but also carbohydrates and other diet ingredients since even ingredients like rice, corn, peas, potatoes, or brewer’s yeast contain protein, which is the component that triggers food allergies.  Since oils, vitamins, and minerals don’t contain protein (and are critical for a nutritionally balanced diet), previous exposure to these ingredients typically isn’t considered for novel ingredient diets.  Even though there are many “novel ingredient” or “limited ingredient” diets available over-the-counter, I recommend only using a veterinary novel ingredient diet for an elimination diet trial because several studies have shown high rates of contamination with common proteins in over-the-counter “allergy” or “limited ingredient” diets and many over-the-counter “limited ingredient” diets contain more ingredients than their name implies.

If your pet’s diet history is incomplete (for example, if you adopted your dog or cat as an adult) and you don’t know everything they’ve ever eaten or if your pet has tried foods with many different ingredients, it can be impossible to find a diet that you’re sure will be novel to your pet. In these cases, a novel ingredient diet is not the best option. Another limitation of using novel ingredient diets is that there may be cross-reactions between proteins from different sources.  Therefore, in many cases, the best option for an elimination diet trial is a hydrolyzed or elemental diet.

2. Hydrolyzed or elemental diets

A hydrolyzed diet’s proteins are broken down (“hydrolyzed”) to be too small to be detected by the body’s immune system.  For elimination diet trials in pets with skin disease, most veterinary dermatologists recommend using an extensively hydrolyzed diet in which the proteins are broken down to be even smaller than in typical hydrolyzed diets, but your veterinarian can help you decide on the best hydrolyzed diet option for your pet’s diet trial.  Elemental diets are another option for the elimination diet trial because they’re made from amino acids, the very small building blocks of protein, along with purified carbohydrate sources (plus important vitamins and minerals).

Selecting the diet

It is very important to talk to your veterinarian about which specific diet is most appropriate for an elimination diet trial for your pet based on their diet history, age (very few novel ingredient or hydrolyzed diets meet the special nutritional needs for growing puppies or kittens), symptoms, other medical conditions, body condition score, and a variety of other factors.  The novel ingredient and hydrolyzed/elemental diets vary widely in terms of their protein, fat, fiber, phosphorus, sodium, and other nutrient levels so the same one is not right for every pet.  I don’t recommend homecooked diets for elimination diet trials unless they are formulated by a Board Certified Veterinary Nutritionist^® to ensure they meet all the pet’s essential nutrient requirements.

Important things that you may be forgetting

Selecting your pet’s main pet food with the help of your veterinarian is critical, but the diet is only one of the many things you’ll need to think about to do a successful elimination diet trial.  The successful elimination diet trial has to control everything that goes into your pet’s mouth, such as treats, rawhides, dental chews and toothpaste, table food, dietary supplements, and flavored medications.  If you don’t also control all of these other things, you can completely invalidate the elimination diet trial, waste a lot of time and effort, and prolong the time until you find the cause of your pet’s skin or digestive issues (which, most often, is something other than a food allergy)!  You can find a downloadable checklist of all the things to think about for a successful elimination diet trial at this link (Elimination diet checklist).  It also can be very helpful to keep a journal during the elimination diet trial to record your pet’s symptoms and if any food missteps occur.

This may seem like a lot to think about but these are all very common pitfalls I’ve seen over the years in pets going through elimination diet trials.  Given the challenges of doing these diet trials, it’s best to make sure it’s done the right way so it only needs to be done once.  Thinking about these common mistakes ahead of time can help you avoid them.  Just remember that it’s only for a limited period of time and the more diligent you are during this trial, the sooner you can help your pet to start feeling better!

Lisa M. Freeman, DVM, PhD, DACVIM (Nutrition)

Dr. Freeman is a veterinary nutritionist and a professor at Cummings School of Veterinary Medicine at Tufts University. She is on the cutting-edge of science, with hundreds of articles in prestigious journals, speaking engagements at national and international conferences, and awards for her scientific achievements. However, she also is passionate about providing objective and accurate information on pet nutrition to veterinarians, pet owners, and other animal enthusiasts.

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